Add support for hierarchical trees

[brylie]

It would be nice if this charting package could support hierarchical trees.

Goal

Add support for hierarchical trees, by incorporating a charting library/component that provides tree layouts.

Possible solution

Merge django-mptt-graph to draw hierarchical trees

[brylie]

We discussed, and agreed (in issue #3), that the django-mptt-graph project could remain in its own repository and just use django-chartflo as a dependency.